Emissions from fossil fuel combustion facilities, such as flue gases of coal-fired utilities and municipal solid waste incinerators, typically include multiple types of gases. For example, emissions from a smokestack can include gases such as CO2, NO2, SO2, etc.
Many countries regulate emissions of the different types of waste gases because of potential environmental hazards posed by such harmful emissions. Accordingly, many facilities that generate or potentially generate harmful gas emissions need to employ multiple gas analyzer systems to ensure that emitted gas concentrations are compliant with corresponding regulations.
To detect the presence of the many types of gases, a facility may need to operate multiple independent conventional gas analyzer systems and/or measurement benches. For example, a facility may need to operate a first gas analyzer system to detect a first type of gas, a second analyzer system to detect a second type of gas, and so on. Such instruments may combine multiple complex analytical technologies including Electrochemical cells, Chemi-luminescence Spectroscopy, Flame Ionization, GFC (Gas Filter Correlation), NDIR (Non-Dispersive Infrared), or UV (Ultra-Violet) Spectroscopy, etc., into a single gas analyzer unit to detect one or more types of gases.
Each of the different types of gases emitted by a smokestack has unique light absorption characteristics. For example, each gas type can absorb different optical frequencies. The unique absorption characteristics enable a corresponding gas analyzer system to identify whether a particular type of gas is present in a gas sample.
A facility may need to operate multiple independent conventional gas analyzer systems and/or measurement benches to detect a presence of multiple gases of interest. Each conventional gas analyzer system typically requires its own set of operating procedures, calibration procedures, etc. to collect accurate data.
One way to identify a type of gas present in an unknown gas sample is the application of Beer's law. In general, Beer's law defines a relationship that relates the absorption of light to properties of the material through which the light is traveling. In other words, as mentioned above, different materials absorb different frequencies of light energy. Based on the passage of optical energy through a gas sample and subsequent detection of the frequencies of optical energy that are absorbed by the gas sample, it is possible to determine what type of gas is present in the gas sample. For example, the amount of absorption by a sample can indicate the concentration of a respective gas.
A conventional gas analyzer system includes an optical source that generates an optical signal for passing through a sample gas. Such a conventional analyzer can include a so-called optical filter wheel and a so-called chopper wheel. The optical filter wheel and the chopper are both disposed in the path of the optical signal.
The optical filter wheel can include a number of different optical filters, each of which passes only a single, narrow frequency band of optical energy. Depending on which filter is disposed in the path of the optical signal, it is known what frequency band of light is being passed through the sample. An optical detector measures how much optical energy passes though the sample.
The chopper wheel includes multiple windows or cut-outs separated by opaque regions that block light. As mentioned, the chopper wheel is also placed in the path of the optical signal such that a position of the chopper wheel dictates whether any of the optical energy passes through the gas sample or is blocked by an opaque region. As the chopper wheel spins, it blocks and passes optical energy of a particular frequency band through the sample to a detector.
During operation, a conventional gas analyzer system produces modulated light by setting the filter wheel in a position so that the optical signal passes through a selected filter in the optical wheel. When the selected filter is in such a position, a controller spins the chopper wheel to repeatedly block and pass the optical signal through the gas sample as discussed above. Application of the chopper wheel results in the modulation of a single frequency band of optical energy depending on which filter on the filter wheel has been chosen to be “chopped” or modulated. Accordingly, a controller can produce a modulated optical signal using a two-wheel assembly including a chopper wheel and filter wheel.